Dr. Sundeep Mishra, a noted professor of Cardiology, argues that while oxygen therapy may seem like the best way to deliver supplemental oxygen to patients, doctors have to decide on its administration after carefully considering evidence that it will indeed be useful and necessary for the patient.

Supplemental oxygen therapy has been used in clinical medicine for more than a century now. It finds wide application even in modern medicine and as a matter of fact, there is a tendency to start delivering oxygen to any patient who is sick. This therapy has become so ingrained in medical science that even lay people expect, nay even demand that their patient be administered this therapy.

In cardiac patients particularly, acute myocardial infarction (AMI) aka heart attack and heart failure represent some of the sickest populations known in the field of medicine. So, it is but natural to expect/deliver supplemental oxygen in these groups of patients.

While this therapy is certainly useful for patients with hypoxaemia (peripheral oxygen saturation <90% or partial arterial oxygen pressure <60 mmHg), where is the evidence for a normoxemic patient?

We now live in an era of evidence-based medicine and therefore evidence of each and every bit of therapy must be investigated however logical it may seem otherwise.

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Mechanism of Action

Oxygen therapy is believed to be life-saving in a variety of clinical situations, particularly in cases with a medical emergency, where its use has practically become a knee-jerk reflex reaction. In context with acute MI (AMI), wherein patients have compromised myocardial perfusion which could lead to myocardial hypoxia there could be a benefit in correcting this hypoxaemia.

Indeed, delivering oxygen in this condition appears logical, an attempt to improve oxygenation of ischemic myocardial muscle, reducing infarct size and perhaps other complications associated with it. However, in reality, there is very little evidence backing it, the practice entirely based on tradition, expert opinion or at best anecdotal evidence.

On the other hand, the beneficial effects may be counter-balanced by increased production of reactive oxygen species and the related oxidative stress (contributing to re-perfusion injury) and by the paradoxical reduction in coronary blood flow and myocardial oxygen consumption resulting from hyperoxia-induced vasoconstriction in the cerebral, coronary, and systemic vasculature.

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Acute Coronary Syndrome

Thus, in this situation actually inadvertent hyperoxia commonly occurs partly because of concerns to ensure sufficient oxygenation and partly because hyperoxia is not perceived to be detrimental.

While early experimental and a few small clinical studies did suggest some benefit of this strategy, most non-randomised studies have revealed that oxygen therapy in reality actually increases myocardial ischaemia, increases infarct size, and even possibly increases mortality. Even the RCTs and subsequent Cochrane analysis based on them failed to show any benefit of routine use of inhaled oxygen in acute heart attack.

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Heart Failure

Heart failure remains an extremely high-risk spectrum among cardiac patients with limited long-term options. Oxygen therapy is utilised in these patients despite the surprisingly limited data that is available. Only 3 small studies utilising oxygen therapy have been undertaken in patients with chronic HF.

1. In one of them, Miak and colleagues studied the effect of oxygen therapy in patients with stable CAD (n = 12) and those with HF (n = 16). They found that extreme hyperoxia (fraction of inspired oxygen [FiO2] = 100%) was again paradoxically associated with impairment of cardiac relaxation and increased left ventricular filling pressures in both the group of patients studied. Furthermore, in the HF group this hyperoxia was associated with several negative haemodynamic changes, such as an increase in systemic vascular resistance, and reductions in cardiac output, stroke volume, and coronary sinus blood flow.
2. In another study Park and colleagues studying 13 patients of HF who were delivered high-flow oxygen (∼5 l/min, FiO2 = 40%) also had a reduction of both cardiac output and heart rate. They also found a trend towards increased systemic vascular resistance without any change in stroke volume compared with room air delivery (FiO2 = 21%).

Thus both studies revealed that that delivery of excessive supplemental oxygen can actually be detrimental to cardiac function.

3. Lastly, a study by Haque and colleagues also revealed reductions in stroke volume and cardiac output and an increase in pulmonary capillary wedge pressure with achieved hyperoxia in patients with ADHF and mild hypoxaemia (baseline peripheral oxygen saturation = 92.6%). Furthermore, these haemodynamic dysfunctions started early with a delivery of as little as 1 l/min of supplemental oxygen (24% FiO2). These data pertain to patients of HF with reduced ejection fraction but are unlikely to be any different even in patients of HF with preserved ejection fraction.

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Acute decompensated heart failure (ADHF)

It represents patients who are even sicker and the benefit of supplemental oxygen therapy is likely to be more compared to chronic HF. Furthermore, herein the treating physicians have even fewer therapeutic options of proven therapies at their disposal and thus rely heavily on what has been classically done down the years i.e. follow the already established clinical practice.

Diuretic agents and oxygen form the backbone of the therapeutic endeavour despite the fact that there is no clear-cut data supporting their utility.

Some surveys have revealed that supplemental oxygen therapy has been prescribed in at least half of the patients with ADHF in causality irrespective of the baseline oxygen saturation level.

However, although the practice of supplementing oxygen in patients with ADHF is common, there have been no published cohort studies or RCTs establishing the role of hyperoxia in patients with ADHF.

Thus, considering the findings (primarily in chronic HF patients), the recent practice guidelines are diverging from the previous consensus that oxygen should be administered routinely in patients with cardiac disease and now advocate using it in only in carefully selected patients.

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Conclusion

Routine administration of supplemental oxygen is not useful in cardiology patients however, it may be useful in patients with hypoxaemia (oxygen saturation <90%).

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

The writer, Dr. Sundeep Mishra is a Professor of Cardiology.